Eliminating special characters of a string stops at spaces

Question

I'm a beginner with C but am trying to make a script which reads input and, ignoring special characters and spaces, outputs the reverse of the input whether or not the letters make a palindrome.

I've tried tweaking the length of the loop in the fix function as I think that's where the issue is but from what I can tell, strlen() is working as expected, the loop is just stopping when it encounters a space.

#include <stdio.h>
#include <string.h>
#include <ctype.h>

#define MAX 1000 /* The maximum number of characters in a line of  
input 
*/
void fix(char str[], char str2[]);
int main()
{
  char text[MAX], text2[MAX], text3[MAX], text4[MAX], temp;
  int i, j;
  int length;

  puts("Type some text (then ENTER):");

  /* Save typed characters in text[]: */

  fgets(text, MAX, stdin);
  length = strlen(text) - 1;

  strcpy(text2, text);

  i = 0;
  j = length-1;
  while(i < j){
        temp = text[i];
        text[i] = text[j];
        text[j] = temp;
        i++;
        j--;
  }
  /* Analyse contents of text[]: */
  printf("Your input in reverse is:\n");
  printf("%s", text);

  fix(text, text3);
  printf("%s\n", text3);
  fix(text2, text4);
  printf("%s\n", text4);
  if(strcmp(text4, text3) == 0)
        printf("Found a palindrome!\n");
return 0;
}


void fix(char str[], char str2[]){
  int i;
  for(i = 0; i < strlen(str)-1; i+=1){
        if (isalpha(str[i])){
                str2[i] = tolower(str[i]);
        }
  }
}

For an input of "Nurses, run", the reversed string outputs correctly, but there is no output of "Found a palindrome!"

Printing text 3 and 4 prints "nur" and "nurses," respectively.

It seems the loop stops when it encounters a space but I can't figure out why as it should go based on the length of the full input.

Answer 1

There is a problem in your fix function that is causing undefined behavior because you only initialize positions in the resulting string that are alpha characters.

Because your requirement is to ignore non-alpha characters, you need an extra counter. Obviously i is going to point at the wrong character if you have to remove characters from your string. So you need to count how many characters you have actually copied into str2 .

void fix(char str[], char str2[])
{
    int i, copied = 0;
    for(i= 0; str[i] != '\0'; i++)              // See note 1
    {
        if (isalpha(str[i])) {
            str2[copied++] = tolower(str[i]);   // See note 2
        }
    }
    str2[copied] = '\0';                        // See note 3
}

Notes:

1. There is no need to call strlen in every test. In fact, you don't need to call it at all -- this just tests for the string's null-terminator.
2. Notice that copied is incremented here. This guarantees that you won't have gaps if you exclude a character.
3. You must remember to null-terminate your new string.